Centrifugal pumps are often used as drain pumps in washing machines or dishwashers. In actual application, the pump is mounted inside of the machine and activated automatically during operation by the control circuit of the machine, mainly to pump out the water from the machine. The whole operation can be divided into three stages: the starting stage, the full water stage and the air-water stage. The air-water stage is the last stage during which most of the water has already been pumped out and only residual water mixed with air, flows through the pump. In general, the air-water stage is the noisiest phase of operation, with the noise level significantly greater than the noise level of the starting or full water stages.